The present invention relates to a tandem type vacuum booster which operates a brake master cylinder or a clutch master cylinder on a motor vehicle in a boost mode, and more particularly to an improvement of a tandem type vacuum booster in which a piston boss integral with a cylindrical valve case which accommodates a control valve is fastened with screw members to the bottom plate of a coupling cylinder which is extended from the inner peripheral edge of a front booster piston and set in the front end portion of the piston boss, the inner peripheral bead of a front diaphragm which is laid on the rear surface of the front booster piston is inserted between the booster piston and the piston boss, a return coil spring is elastically set between the front end wall of a booster shell and the piston boss, to urge the piston boss backwardly, and an output rod is coupled to the piston boss in such a manner that the output rod is protruded forwardly of the piston boss.
A tandem type vacuum booster of this type has been well known in the background art, for instance, by Examined Japanese Utility Model Publication No. Hei. 4-17415.
If, in the tandem type vacuum booster, a coupling cylinder extended from the inner peripheral edge of a front booster piston is low in rigidity, then the coupling cylinder may be deformed during boost operation. The deformation of the coupling cylinder, adversely affecting various components, may lower the air-tightness of sealing members such as the inner peripheral bead of the front diaphragm.
Hence, in the conventional tandem type vacuum booster, the front booster piston and the coupling cylinder are made of a thick steel plate so that the coupling cylinder is sufficiently high in rigidity. However, the use of the thick steel plate for formation of the front booster piston results in an undesirable increase in weight of the booster.